Real time fax relay, for example, implemented with a T.30 protocol, allows a local facsimile machine to send fax pages over one or more networks to a remote facsimile machine, such that the remote facsimile machine receives the fax pages while the local facsimile machine is still transmitting the fax pages. Due to real time nature of these fax transmissions, the T.30 protocol includes timeout functionality that allows the local facsimile machine to retransmit data from a fax page that was incorrectly received by the remote facsimile machine.
When there is excessive transmission delay over the communication network, the remote facsimile machine will trigger timeouts requesting the local facsimile machine retransmit the delayed data. This transmission delay may be caused when at least one of the networks are bandwidth constrained, or by other inherent properties of the networks. For instance, when one of the networks is an IP network, the total transmission delay increases with demodulation, packetization, and remodulation of fax data as well as the addition of de-jitter buffers. Previously, IP gateways spoofed the local facsimile machine into a “hold off” condition, forcing a postponement of the retransmission of fax data until the transmission delay lessened.
Fax pages are often transmitted through multiple tandemly coupled communication networks, such as a TDM network coupled between IP networks, requiring a conversion from packet encoding to an analog signal that adds to the total transmission delay. The transmission delay due to network conversion further increases with each additional tandemly coupled network. Since the transmission delay due to network conversion is systemically-based according to network configuration, many IP gateways will spoof or “hold off” retransmissions indefinitely thus locking up real time fax relay.